This proposal is for the purchase of a complete single crystal X-ray diffractometer system for data collection and solution and refinement of molecular structures. The system is to consist of a diffractometer equipped with a low-temperature apparatus and a dedicated computer system for all crystallographic calculations. The instrument will be used by the X-Ray Crystallographic Laboratory to provide structure determinations for a wide range of health-related research projects that include: analysis of possible relationships between unsaturated polyolefins and hydrocarbon carcinogenesis; synthesis of cocarcinogens and diterpenes having powerful cytotoxic properties; evaluation of the possible antiviral role of polycondensed alicyclic molecules of high symmetry; synthesis of biologically active alkaloids and amino acid derived natural products; construction and analysis of mode of action of macrocyclic transition metal complexes in biomimetic chemistry; development of chelate dioxygen carriers for use in facilitated transport; biosynthesis of marine natural products and active microbial metabolites; stereochemistry of reactions at prochiral and proprochiral centers; biosynthesis of riboflavin and related compounds; multiple hydrogen bonding to nucleic acid bases; and much more.